The present invention relates to a gas-selective permeable membrane, particularly for leak detectors, and to the method for its manufacturing.
In the field of leak detection in ducts, tanks etc., the use of apparatuses known as “leak detectors” is widespread. Such apparatuses generally comprise a vacuum-tight chamber equipped with a selective membrane through which only a predetermined gas can flow into the chamber, when the pressure inside the chamber is made significantly lower than the outside pressure.
The membranes of the known leak detectors are generally made of quartz or glass with high silica content. Such membranes are permeable to helium if they are brought to a suitable temperature, typically at least 300° C. Use of such membranes has become particularly popular also because helium is a harmless, inert gas that is present in very small amounts in the atmosphere and hence is suitable for use as a test gas for leak detection.
An electrical resistor is generally used to bring the membrane to the temperature at which the membrane material becomes permeable.
The operation of the leak detectors is as follows: once a sufficient vacuum has been created in the chamber, the detector can absorb, through the selective membrane, an amount of the test gas. If the test gas is present in the surrounding environment, for instance because of a leak from a volume into which said gas has been previously introduced, the gas penetrates into the detector chamber from which it is pumped to the outside by the vacuum pump. The presence of test gas within the chamber results in an increase of the electric current drawn by the vacuum pump if compared to vacuum conditions. The increase of the electric current is signalled by a detector informing of the presence of the test gas and, consequently, of a probable leak in the volume to be tested.
To achieve a good sensitivity, the membrane must be very thin, since gas permeability is inversely proportional to the membrane thickness. Moreover, the membrane must resist to high temperatures, since gas permeability is proportional to the membrane temperature.
The membranes presently used generally consist of a capillary tube and the electrical resistor for heating the membrane is helically wound around the capillary tube. A leak detector having a capillary tube membrane is disclosed for instance in patent application No. EP 0352371 “Helium leak detector with silica glass probe”.
Capillary tube membranes however are fragile, and securing the capillary tube to the vacuum line is difficult. Moreover, the capillary tube shape is not satisfactory in terms of sensitivity, since it is impossible to heat the capillary tube surface wholly and uniformly to the ideal temperature for a good permeability to the test gas. This is due in part to the limitations in possibility of increasing the resistor's temperature, and the capillary tube is glued to the vacuum line.
Moreover, the capillary tube shape increases the chamber volume and, consequently, both the response inertia of the detector in the presence of the test gas, and the time necessary to have the detector again operating after a leak has been detected.
Planar membranes have been developed in the past to obviate these drawbacks.
These membranes have a composite structure in which a conventional metallic support layer, providing the structural strength, is associated with a thin layer of a material selectively permeable to the test gas. The support layer, which is of a gas impermeable material, has openings or windows through which the permeable layer is exposed at both faces. An example of such a membrane is disclosed in U.S. Pat. No. 3,505,180, in which a hydrogen-permeable layer of palladium is superimposed to a metal support layer provided with openings.
Yet, also that solution is not wholly satisfactory because of the different physical properties of the materials forming the membrane. For instance, the different thermal expansion coefficients may compromise the membrane life. Moreover, separation phenomena of the different layers forming the composite structure may occur. The latter drawback is very penalising in terms of permeability to the test gas, since it limits the temperature to which the membrane can be heated.
It is a main object of the present invention to provide a selective membrane for leak detectors, allowing overcoming the above drawbacks, as well as a method for producing such a membrane.
It is another object of the present invention to provide a selective membrane for gas detection, having high sensitivity and reliability.
The above and other objects are achieved by a membrane for gas detection according to the invention, as claimed in the appended claims.